(1) Field of the Invention
The present invention relates to a tandem-type image forming apparatus which has image holding components and is able to switch between a full-color copy mode using all of the image holding components to form a full-color image and a monochrome copy mode using only one of the image holding components to form a single-color image.
(2) Related Art
With the widespread use of personal computers in recent years, color documents have become increasingly common in organizations, such as offices. As a result, there are increasingly demands for image forming apparatuses that can perform color printing at high speed.
Tandem-type color copying machines have received much attention as examples of this type of image forming apparatus. In a tandem-type color copying machine, cyan, magenta, yellow, and black toner images are sequentially transferred onto a recording sheet by image forming units. These image forming units correspond to the four colors, are set along a transportation path of a recording sheet, and each include a photosensitive drum and a transfer unit as their main components. The recording sheet is transported by a transport belt that runs over a plurality of rollers. In general, the photosensitive drum used for forming a black image is set at a rearmost position on the transportation path of the recording sheet for better reproduction of black parts of the color image.
When performing operations aside from full-color image formation, such as when forming a black image using this type of image forming apparatus, toner images are not formed on the photosensitive drums for cyan, magenta, and yellow, and a toner image is formed only on the photosensitive drum used for the black image formation.
However, when only one photosensitive drum is used, the recording sheet still comes into contact with the other three photosensitive drums during transportation. For this reason, the three photosensitive drums which are not used for the image formation still need to be rotated. This results in needless wear and tear on the photosensitive drums and cleaning blades that are in contact with the photosensitive drums.
To address this problem, Japanese Laid-Open Patent Applications No. 3-288173, No. 6-258914, and No. 6-102776 teach examples of a color image forming apparatus. These color image forming apparatuses have the transport belt contact all of the photosensitive drums when forming a full-color image (referred to as the "color copy mode" hereinafter), and rotates the whole transport belt about the rotational axis of the photosensitive drum of the image forming unit used for forming a black image when forming a black image (referred to as the "monochrome copy mode" hereinafter) so that the transport belt does not come into contact with the photosensitive drums for cyan, magenta, and yellow which are not used for black image formation.
Accordingly, the photosensitive drums of cyan, magenta, and yellow do not need to be rotated when a black image is formed. This prevents needless wear and tear on these photosensitive drums.
With these conventional image forming apparatuses, the transport belt is separated from the photosensitive drums and shifted downward in the monochrome copy mode. This causes variations in the position at which the recording sheet is held on the transport belt (that is, the position from which the transport belt starts transporting the recording sheet) and variations in the transportation path length. As a result, time taken for the recording sheet to reach the photosensitive drum of the image forming unit used for forming a black image varies, so that an image may be transferred onto the recording sheet at an incorrect position.
In addition, when using an image forming apparatus which selectively operable in the color copy mode and the monochrome copy mode, a user specifies one copy mode using an operation panel, with the transport belt being separated from or pressed against the photosensitive drums before a series of image formations are executed.
For this reason, when performing a copying job for a mixture of color and monochrome documents using an image forming apparatus with an automatic document feeder (ADF), the user sets the color copy mode and presses a copy start key. Here, regardless whether a document is color or monochrome, image formation is conventionally performed for all of the documents in the same copy mode, with the photosensitive drums and the transport belt being in contact with each other.
Regarding business documents in particular, the ratio of monochrome documents in a copying job is considerably high. However, if copying jobs that include a relatively low proportion of color images still need to be performed in the color copy mode, this means that the monochrome mode where needless wear and tear on the unused photosensitive drums is prevented cannot be effectively used. This poses a major constraint to the operational effect of providing such monochrome copy mode.
This problem also occurs when using a printer as the image forming apparatus to successively print out a document including both color and monochrome pages.